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<h1>Transactions</h1>

<p>
In this chapter, we will work with transactions. First, we provide some
basic definitions. Then we will have programs that show, how to work
with transactions. 
</p>

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<p>
A <b>transaction</b> is an atomic unit of database operations 
against the data in one or more databases. The effects of all the SQL statements
in a transaction can be either all committed to the database or all rolled back.
</p>

<p>
In SQLite, any command other than the SELECT will start an implicit transaction. 
Also, within a transaction a command like CREATE TABLE ..., VACUUM, PRAGMA, will 
commit previous changes before executing.
</p>

<p>
Manual transactions are started with the BEGIN TRANSACTION statement and finished 
with the COMMIT OR ROLLBACK statements.
</p>

<p>
SQLite supports three non-standard transaction levels. DEFERRED, IMMEDIATE and EXCLUSIVE. 
SQLite automatically puts each command into its own transaction unless we start our
own transaction. Note that this may be influenced by the driver too. SQLite Python
driver has the autocommit mode turned off by default and the first SQL command
starts a new transaction.
</p>

<pre class="code">
Option Strict On

Imports Mono.Data.Sqlite

Module Example

    Sub Main()

        Dim cs As String = "URI=file:test.db"

        Using con As New SqliteConnection(cs)
        
            con.Open()
        
            Using cmd As New SqliteCommand(con)

                cmd.CommandText = "DROP TABLE IF EXISTS Friends"
                cmd.ExecuteNonQuery()
                cmd.CommandText = "CREATE TABLE Friends(Id INTEGER PRIMARY KEY," _ 
                                    &amp; "Name TEXT)"
                cmd.ExecuteNonQuery()
                cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Tom')"
                cmd.ExecuteNonQuery()
                cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Rebecca')"
                cmd.ExecuteNonQuery()
                cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Jim')"
                cmd.ExecuteNonQuery()
                cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Robert')"
                cmd.ExecuteNonQuery()
                cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Julian')"
                cmd.ExecuteNonQuery()
        
            End Using

            con.Close()

        End Using
        
    End Sub

End Module
</pre>

<p>
We create a Friends table and fill it with data. We do not explicitly start a 
transaction, nor we call commit or rollback methods. Yet the data is written 
to the table. It is because by default, we work in the autocommit mode. 
In this mode each SQL statement is immediately effective.
</p>

<pre class="explanation">
cmd.CommandText = "DROP TABLE IF EXISTS Friends"
cmd.ExecuteNonQuery()
cmd.CommandText = "CREATE TABLE Friends(Id INTEGER PRIMARY KEY," _ 
                    &amp; "Name TEXT)"
cmd.ExecuteNonQuery()
</pre>

<p>
We drop the Friends table if it already exists. Then we create the table with 
the CREATE TABLE statement.
</p>

<pre class="explanation">
cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Tom')"
cmd.ExecuteNonQuery()
cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Rebecca')"
cmd.ExecuteNonQuery()
...
</pre>

<p>
We insert two rows. 
</p>

<pre>
sqlite> SELECT * FROM Friends;
1|Tom
2|Rebecca
3|Jim
4|Robert
5|Julian
</pre>

<p>
The Friends table was successfully created. 
</p>


<hr class="btm"> 

<p>
In the second example we will start a custom transaction with 
the <code>BeginTransaction()</code> method.
</p>


<pre class="code">
Option Strict On

Imports Mono.Data.Sqlite

Module Example

    Sub Main()

        Dim cs As String = "URI=file:test.db"

        Using con As New SqliteConnection(cs)
        
            con.Open()

            Dim tr As SqliteTransaction = con.BeginTransaction()
        
            Using tr 
            
                Using cmd As New SqliteCommand(con)

                    cmd.Transaction = tr
                    cmd.CommandText = "DROP TABLE IF EXISTS Friends"
                    cmd.ExecuteNonQuery()
                    cmd.CommandText = "CREATE TABLE Friends(Id INTEGER PRIMARY KEY," _ 
                                        &amp; "Name TEXT)"
                    cmd.ExecuteNonQuery()
                    cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Tom')"
                    cmd.ExecuteNonQuery()
                    cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Rebecca')"
                    cmd.ExecuteNonQuery()
                    cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Jim')"
                    cmd.ExecuteNonQuery()
                    cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Robert')"
                    cmd.ExecuteNonQuery()
                    cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Julian')"
                    cmd.ExecuteNonQuery()
                    cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Jane')"
                    cmd.ExecuteNonQuery()
                End Using

                tr.Commit()
            End Using

            con.Close()
        End Using
        
    End Sub

End Module
</pre>

<p>
All SQL commands form a unit. Either all are saved or nothing is saved. This is
the basic idea behind transactions.
</p>

<pre class="explanation">
Dim tr As SqliteTransaction = con.BeginTransaction()
</pre>

<p>
The <code>BeginTransaction()</code> method starts a transaction.
</p>

<pre class="explanation">
cmd.Transaction = tr
</pre>

<p>
We set the transaction within which the <code>SqliteCommand</code> executes.
</p>

<pre class="explanation">
tr.Commit()
</pre>

<p>
If everything ran OK, we commit the whole transaction to the database.
In case of an exception, the transaction is rolled back behind the scenes. 
</p>


<h2>Explicit rollback call</h2>

<p>
Now we are going to show an example, where we rollback manually a transaction, 
in case of an exception.
</p>

<pre class="code">
Option Strict On

Imports Mono.Data.Sqlite

Module Example

    Sub Main()

        Dim cs As String = "URI=file:test.db"

        Dim con As SqliteConnection     
        Dim tr As SqliteTransaction
        Dim cmd As SqliteCommand
        
        Try
            con = New SqliteConnection(cs)
        
            con.Open()
    
            tr = con.BeginTransaction()
            cmd = con.CreateCommand()
                
            cmd.Transaction = tr
            cmd.CommandText = "DROP TABLE IF EXISTS Friends"
            cmd.ExecuteNonQuery()
            cmd.CommandText = "CREATE TABLE Friends(Id INTEGER PRIMARY KEY," _
                              &amp;  "Name TEXT)"
            cmd.ExecuteNonQuery()
            cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Tom')"
            cmd.ExecuteNonQuery()
            cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Rebecca')"
            cmd.ExecuteNonQuery()
            cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Jim')"
            cmd.ExecuteNonQuery()
            cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Robert')"
            cmd.ExecuteNonQuery()
            cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Julian')"
            cmd.ExecuteNonQuery()
            cmd.CommandText = "INSERT INTO Friends(Name) VALUES ('Jane')"
            cmd.ExecuteNonQuery()
                
            tr.Commit()

        Catch ex As SqliteException
        
            Console.WriteLine("Error: {0}",  ex.ToString())

            If tr IsNot Nothing
                Try                 
                    tr.Rollback()                    
                Catch ex2 As SqliteException
                    Console.WriteLine("Transaction rollback failed.")
                    Console.WriteLine("Error: {0}",  ex2.ToString())
                Finally                
                    tr.Dispose()
                End Try
            End If
        Finally 
            If cmd IsNot Nothing
                cmd.Dispose()
            End If

            If tr IsNot Nothing
                tr.Dispose()
            End If

            If con IsNot Nothing
                Try                 
                    con.Close()
                Catch ex As SqliteException                
                    Console.WriteLine("Closing connection failed.")
                    Console.WriteLine("Error: {0}",  ex.ToString())
                Finally                
                    con.Dispose()
                End Try
            End If
        End Try    
        
    End Sub

End Module
</pre>

<p>
We create our own Try, Catch, Finally blocks, where we deal with
possible issues.
</p>

<pre class="explanation">
Catch ex As SqliteException

    Console.WriteLine("Error: {0}",  ex.ToString())

    If tr IsNot Nothing
        Try                 
            tr.Rollback()                    
        Catch ex2 As SqliteException
            Console.WriteLine("Transaction rollback failed.")
            Console.WriteLine("Error: {0}",  ex2.ToString())
        Finally                
            tr.Dispose()
        End Try
    End If
</pre>

<p>
When an exception is thrown during the creation of the Friends table,
we call the <code>Rollback()</code> method. Rolling back a transaction
might fail too; we check this scenarion.
</p>

<pre class="explanation">
If cmd IsNot Nothing
    cmd.Dispose()
End If

If tr IsNot Nothing
    tr.Dispose()
End If
</pre>

<p>
When all goes OK, we dispose the resources. 
</p>

<pre class="explanation">
If con IsNot Nothing
    Try                 
        con.Close()
    Catch ex As SqliteException                
        Console.WriteLine("Closing connection failed.")
        Console.WriteLine("Error: {0}",  ex.ToString())
    Finally                
        con.Dispose()
    End Try
End If
</pre>

<p>
When closing a connection, we might receive another exception. 
We handle this case here.
</p>


<h2>Errors</h2>

<p>
When there is an error in the transaction, the transaction is rolled back 
an no changes are committed to the database.
</p>

<pre class="code">
Option Strict On

Imports Mono.Data.Sqlite

Module Example

    Sub Main()

        Dim cs As String = "URI=file:test.db"

        Using con As New SqliteConnection(cs)
        
            con.Open()

            Dim tr As SqliteTransaction = con.BeginTransaction()
        
            Using tr 
        
                Using cmd As New SqliteCommand(con)

                    cmd.CommandText = "SELECT * FROM Cars LIMIT 5"
                    
                    cmd.Transaction = tr
                    cmd.CommandText = "UPDATE Friends SET Name='Thomas' WHERE Id=1"
                    cmd.ExecuteNonQuery()

                    cmd.CommandText = "UPDATE Friend SET Name='Bob' WHERE Id=4"
                    cmd.ExecuteNonQuery()
                    
                    tr.Commit()
                End Using
            End Using
            con.Close()
        End Using
        
    End Sub

End Module
</pre>

<p>
In the code example we want to change two names. There are two statements 
which form a transaction. There is an error in the second SQL statement. 
Therefore the transaction is rolled back.
</p>

<pre class="explanation">
cmd.CommandText = "UPDATE Friend SET Name='Bob' WHERE Id=4"
cmd.ExecuteNonQuery()
</pre>

<p>
The name of the table is incorrect. There is no Friend table in 
the database.
</p>

<pre>
$ mono error.exe 

Unhandled Exception: Mono.Data.Sqlite.SqliteException: SQLite error
no such table: Friend
...
</pre>

<p>
Running the example will display this error message. The transaction is rolled back.
</p>

<pre>
sqlite> SELECT * FROM Friends;
1|Tom
2|Rebecca
3|Jim
4|Robert
5|Julian
6|Jane
</pre>

<p>
No changes took place in the Friends table. Even if the first UPDATE 
statement was correct.
</p>

<hr class="btm"> 

<p>
We will again try to change two rows; this time without using the
<code>SqliteTransaction</code>.
</p>

<pre class="code">
Option Strict On

Imports Mono.Data.Sqlite

Module Example

    Sub Main()

        Dim cs As String = "URI=file:test.db"

        Using con As New SqliteConnection(cs)
        
            con.Open()
        
            Using cmd As New SqliteCommand(con)

                cmd.CommandText = "SELECT * FROM Cars LIMIT 5"
                
                cmd.CommandText = "UPDATE Friends SET Name='Thomas' WHERE Id=1"
                cmd.ExecuteNonQuery()

                cmd.CommandText = "UPDATE Friend SET Name='Bob' WHERE Id=4"
                cmd.ExecuteNonQuery()
                
            End Using

            con.Close()
        End Using
        
    End Sub

End Module
</pre>

<p>
We try to update two names in the Friends table. Tom to Thomas and
Robert to Bob. 
</p>

<pre class="explanation">
cmd.CommandText = "UPDATE Friend SET Name='Bob' WHERE Id=4"
cmd.ExecuteNonQuery()
</pre>

<p>
This UPDATE statement is incorrect.
</p>


<pre>
$ mono error2.exe 

Unhandled Exception: Mono.Data.Sqlite.SqliteException: SQLite error
no such table: Friend
...
</pre>

<p>
We receive the same error message as in the previous example.
</p>

<pre>
sqlite> SELECT * FROM Friends;
1|Thomas
2|Rebecca
3|Jim
4|Robert
5|Julian
6|Jane
</pre>

<p>
However this time, the first UPDATE statement was saved. The second one
was not. 
</p>

<p>
In this part of the SQLite Visual Basic tutorial, we have worked with transactions.
</p>

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